The present invention relates to a cryogenic station for delivering a cryogen substantially free of higher boiling impurities such as carbon dioxide and moisture. More particularly, the present invention relates to such a cryogenic station in which the cryogen is vaporized within an auxiliary tank under equilibrium conditions such that the higher boiling impurities are substantially retained within the cryogen in a solidified state and the cryogenic vapor formed upon vaporization of the cryogen is thereby substantially free of the higher boiling impurities.
A requirement exists in various industries for storage and delivery of cryogenic gases free of higher boiling impurities. This need is particularly acute in the electronics industry. In the electronics industry, various ultra-high purity atmospheric gases are used in the manufacture of semiconductor devices. Such ultra-high purity atmospheric gases are stored in a liquid cryogenic state within cryogenic stations for eventual delivery as a cryogenic vapor. Such cryogenic stations consist of an insulated storage tank having a pressure building circuit. Liquid from the bottom of the storage tank is vaporized in a pressure building coil of the pressure building circuit and the vapor is subsequently introduced into the head space of the tank in order to pressurize the tank to a delivery pressure. Gaseous product, initially in the form of a cryogenic liquid, is delivered from the tank at the delivery pressure to an ambient temperature heat exchanger in order to vaporized the product and warm it to ambient temperature.
The higher boiling impurities, that is impurities that boil at temperatures above the boiling temperature of the cryogen, such as moisture and carbon dioxide, initially solidify within the cryogen, but eventually vaporize along with the cryogen. As a result, the impurities are delivered from the tank along with the product.
As will be discussed hereinafter, the present invention provides a cryogenic station for delivering a cryogen substantially free of the higher boiling impurities.